Mesh bisect with fill doesn't place triangles for the cut surface
The "Bisect" function did generate a surface at the cut area, though it's an N-gon, STL-export seems to not offer an option to triangulate N-gons automatically. 

Solution:

You can select the cut surface in Edit mode with Face selection and then use "Triangulate Faces" in the "Face" Menu Ctrl+F. Or use the direct shortcut, in this case Ctrl+T.

Here an example:



This should do the trick.